Get Volunteer Canola Under Control

Canola has truly been the Cinderella crop for prairie agriculture. Unfortunately, there is a downside to this story. The increase in production and its overall management has also led to real challenges for farmers today — namely volunteer canola.

Agriculture and Agri-Food Canada’s 2014-15 Saskatchewan weed survey found volunteer canola to be the fourth most prevalent weed in the province. That is up from 14th place back in 2003, passing Canada thistle, cleavers and even lamb’s quarters.

Jeanette Gaultier, weed specialist with Manitoba agriculture, reports that the 2016 weed survey in that province has identified volunteer canola to be the fifth most common weed, up from ninth place in the previous survey.

This is backed up by the 2015 National Weed Survey, which, with input from over 700 weed scientists from across Canada and the U.S., lists volunteer canola as the most common and most troublesome weed in Manitoba soybean fields, and the third most common and most troublesome weed in Alberta pulse crops.

With the estimated value of canola harvest losses on the prairies at about a billion dollars annually, how did volunteer canola become such a problem and how do producers control volunteer canola?

A paper recently published in the journal Crop Science by a team of leading Canadian plant scientists

describes a three-year study of canola harvest losses on 310 western Canadian farms. It found crop losses ranging from a low of 2.3 per cent on a farm in Lacombe, AB, to a high of 11.2 per cent on a Saskatoonarea farm. The average yield loss across all 310 farms was 5.9 per cent.

Aside from the obvious income loss of canola left on the ground, farmers are also adding a massive number of seeds to the soil seed bank. A 5.9 per cent harvest loss on a 50-bushel canola crop means 2.95 bu./ ac. left on the ground. That’s equivalent to a seeding rate of just under 150 lb./ac., or 30 times the traditional canola seeding rate.

Imagine if that lost canola had a thousand kernel weight (TKW) of four grams, you’d be leaving about 17 million canola seeds per acre on the field (390 seeds per square foot).

So is it any wonder why volunteer canola has become such a big weed problem? One made worse by the fact that canola seed can remain viable for a number of years.

Controlling volunteers can be tricky considering losses due to shatter (wind, hail, etc.) and regular losses at harvest that lead to an increase in the seed bank. When compounded with shorter rotations, even fewer options remain to control volunteers.

Steve Shirtliffe, a professor in the Department of Plant Sciences at the University of Saskatchewan, says the volunteer canola problem is getting worse given higher canola yields, shorter rotations and the ability of canola seed to develop dormancy.

Back in 2003, he began calling for work into screening canola varieties for their propensity to develop dormancy. “Now, 15 years later, work is finally underway to try to identify molecular markers that would allow selection of varieties by their dormancy,” he says.

MANAGEMENT OPTIONS

“Reducing harvest losses is the first step in controlling volunteer canola,” says Shirtliffe. One way to help reduce those losses is to use pod shatter varieties such as InVigor L140P or InVigor L233P, which have shown in DSTs to reduce harvest seed loss by up to 62 per cent over regular canola varieties. Another way is to attempt to reduce seed loss at harvest.

But are growers fully aware of the extent of their harvest losses? A combine loss monitor, even if properly and accurately set, reflects changes in the rate of loss only. It can’t tell you how much you’re actually losing.

To calculate that, you need to use a collection pan under the combine to catch the seed, then measure that by weight or volume to determine actual bushels per acre losses. The Canola Council of Canada has a step-by-step process online. Visit www.farmforum.ca/measurecombinelosses for details.

Remember too, harvest losses are not limited to the combine and can occur when swathing or at the header if straight cutting.

Reducing harvest losses can greatly reduce volunteers, but even the best harvest management system will still leave at least 0.5 bu./ac., or about 60 seeds per square foot, behind — still well above the typical seeding rate. The good news is not all of these seeds will be viable and some will germinate in the fall and die come winter.

Then there’s tillage. Charles Geddes, a graduate student at the University of Manitoba, researched the impact of various tillage practices on controlling volunteers in southern Manitoba. He found fall soil disturbance could significantly reduce the number of viable volunteer canola seeds in the soil seed bank.

Geddes work contradicts research in England, Germany and Austria, which found the greatest seed bank reduction in those countries happened with zero tillage. More study is needed to determine if the seed bank depletion Geddes noted in Manitoba is climate related or a result of differences in tillage depth, seed predation and/or seed decay between Manitoba and Europe.

Even with the best harvest management practices and careful tillage routines, there will still be plenty of volunteer canola the following year, meaning some type of chemical control is usually required. What to use largely depends on your crop rotation plan. Cereal after canola offers quite a few broadleaf herbicide options for controlling most canola system volunteers. The choices drop off if you’re following with a specialty crop.

Pay close attention to which canola system you grew the previous year. A glyphosate burn off will not control glyphosate-resistant volunteers, so you’ll have to add a broadleaf product for pre-seed volunteer canola control.

Keep good records of your crop rotation and herbicides used. Be aware of the potential for outcrossing of herbicide resistance traits. For example, if a field has only ever been seeded to the LibertyLink system, that doesn’t necessarily eliminate any outcrossing with neighbouring Clearfield or Roundup Ready fields, so you may miss some volunteers if you just rely on one chemistry for control.

Finally, be vigilant about controlling volunteers on field edges and watch for spray misses. A single plant can produce up to 500 seeds and just adds to the volunteer problem. More importantly, unchecked volunteers act as hosts for disease and insects. FF

*Paper: Evaluation of the Causes of On-Farm Harvest Losses in Canola in the Northern Great Plains, Published May 2016 in Crop Science by: Andrea Cavalieri, Department of Plant Science, University of Manitoba; Neil Harker, Agriculture and Agri-Food Canada, Lacombe AB; Linda Hall, Department of Agricultural, Food and Nutritional Science, University of Alberta; and Chris Willenborg, Department of Plant Science, University of Saskatchewan.